Ammonia in aqueous solution is present as an equilibrium system defined by:NH4+τNH3+H+with an equilibrium constant of:       K    a    =                              [                      NH            3                    ]                ⁡                  [                      H            +                    ]                            [                  NH          4          +                ]              =          5.848      ×              10                  -          10                    at 20° C. Where [NH3] represents the concentration of dissolved neutral ammonia. Techniques available for the removal of ammonia from aqueous streams can normally only recover either the ionic [NH4+] or gaseous form of ammonia [NH3]. For efficient removal, adjusting the pH of the aqueous stream to a pH less than 7 or more than 11, maximizes the concentration of either the ionic or gaseous form of ammonia respectively. In actual practice, to maximize the concentration of gaseous ammonia, the pH is typically adjusted to a value greater than 11 using lime or sodium hydroxide.
The gaseous form of ammonia can be removed from water by air stripping where it is contacted with large volumes of air. As the volatility of ammonia increases with temperature, the current state-of-the art of air stripping occurs at higher temperatures. Many configurations of contacting equipment have been used, including countercurrent and crosscurrent stripping towers, spray towers, diffused aeration, and stripping ponds with and without agitation. The ammonia has been recovered from the air by contacting the ammonia-laden air with sulfuric acid solution to form a solution of ammonium sulfate.
Steam stripping has also been used commercially, especially in the removal of ammonia from sour waters. As with air stripping, steam stripping typically involves adjusting the pH to levels greater than 11 using lime or sodium hydroxide. One process for treating petroleum sour waters uses steam stripping which with further downstream processing results in the recovery of ammonia in an anhydrous form, see Leonard et al., “Treating acid & sour gas: Waste water treating process”, Chemical Engineering Progress, October, (1984), pp. 57-60. Mackenzie and King, “Combined solvent extraction and stripping for removal and isolation of ammonia from sour waters”, Industrial Eng. and Chem. Research, 24, (1985), pp. 1192-1200, have examined the combined use of steam stripping and solvent extraction for the removal of ammonia from sour waters with reduced steam consumption.
Cation exchange and zeolites have been used to recover the ammonium form of ammonia from aqueous streams, see for example Berry et al. “Removal of Ammonia From Wastewater”, U.S. Pat. No. 4,695,387 (1987), and Wirth, “Recovery of ammonia or amine from a cation exchange resin”, U.S. Pat. No. 4,263,145 (1981). For these uses the pH is typically adjusted to lower than neutral levels. Temperature plays a much less significant role than in stripping. The cation exchange resins or zeolites are then regenerated by treatment with metal hydroxide solutions to give gaseous ammonia for which the resins and zeolites have no affinity.
References in the literature appear for the use of liquid membranes, hollow fibers, and reverse osmosis to remove ammonia from aqueous streams, although none of these techniques have apparently been commercialized.
Ligand exchange adsorption has been used to recover ammonia. In ligand exchange adsorption, an ion exchange resin is loaded with a complexing metal ion such as Cu2+, Zn2+, Ni2+, Ag+, etc. (Helfferich, F., Ligand Exchange, I & II, Jnl. of the Am. Chem. Soc., No. 84, pp. 3237-3245, 1962). The metal ion then acts as a solid sorbent for ligands such as ammonia. In theory, each metal ion may adsorb a number of ligands up to its coordination number, normally 4 to 6. In practice, not all of these sites will be occupied by an ammonia molecule.
When applied to ammonia, ligand exchangers will only form complexes with the uncharged form of the ammonia. Dawson, in U.S. Pat. No. 3,842,000 (1974) applied ligand exchange to the removal of ammonia from aqueous streams. Dawson used Cu2+ as the metal ion because of its high amine complex formation constant and Dowex™ A-1 as the ion exchange resin. Ammonia was adsorbed after adjusting the pH of the solution to 9-12 to increase the availability of dissolved gaseous ammonia. Contacting the ligand exchange resin with a solution of sulfuric, nitric, phosphoric, or hydrochloric acid regenerated the ligand exchange resin. However, metal is stripped from the resin with each regeneration when a strong acid is used (see immediately below).
Dobbs et al. in “Ammonia removal from wastewater by ligand exchange”, Adsorption and Ion Exchange, AIChE Symposium Series, 71(152), (1975), pp. 157-163, examined the use of dilute hydrochloric acid and Jeffrey, M., Removal of ammonia from wastewater using ligand exchange, M. S. Thesis, Louisiana State University, (1977) (see Regeneration pp. 72-79), examined the use of dilute sulfuric acid as a regenerate for a Cu2+ ligand exchange resin. Both dilute hydrochloric acid and dilute sulfuric acid were found to be ineffective as they leached the copper from the resin at unacceptably high levels. Both Jeffrey (1977) and Dobbs et al. (1975, 1976) attempted to use heat to remove the ammonia from the ligand exchange resin. Jeffrey's use of warm water up to 45° C. removed some ammonia, but failed to prove an effective regeneration agent. Dobbs et al. (1975, and in U.S. Pat. No. 3,948,842) used 30 psig (21,000 kg/m2) steam as a regeneration agent. Although successful in regenerating most of the ligand exchange resins activity, the process was energy intensive and produced peak ammonia concentrations in the condensed steam of only 800 ppm.
An object of the invention is to provide an ammonia recovery process that is more economical than current methods for removal of ammonia from fluid streams.
Another object of the invention is to provide an ammonia recovery process that uses fewer chemicals than current processes or chemicals compatible with the original process application. Typically this involves regeneration and recycle of the sorbent material(s).
Another object of the invention is to reduce ammonia concentration in the effluent stream to very low levels (i.e. less than or equal to 10 ppm) or to control the ammonia concentration to meet environmental regulations.